Compression and expansion of range of energy level



Aug. 30, 1932. 1.. c. ROBERTS 1,375,157

COMPRESSION AND EXPANSION OF RANGE OF ENERGY LEVEL Filed June 29, 1929D'amnglksion INVENTOR Z. a Rolfrlis ATTOR N EY Patented Aug. 30, 1932 inSTATES PATENT Fries LELAND C. ROBERTS, 0F TOWAC'O, NEW JERSEY, ASSIGNORT0 AMERICAN TELEPHONE AND TELEGRAPH COMPANY, AOORPORATION OF NEW YORKCOMPRESSION AND EXPANSION OF RANGE OF ENERGY LEVEL Application filedJune 29,

rangement of an amplifier and of two reistances connected in series, oneof the resistances having a low temperature coefficient and the other ahigh temperature coefiicient. This use of resistances having differenttemperature coefiicients is disclosed in a different arrangement in myapplication filed of even date herewith and given Serial No. 374,919.

The present invention will be clearly understood when the followingdetailed description is read with reference to the accompanying drawing,which shows diagrammatically the circuit arrangements involved.

As indicated above, in the transmission of broadcast programs overtelephone circuits, it is often desirable to compress the range ofenergy level of the currents in the telephone circuit. This may beaccomplished by causing the currents of high energy level to beamplified less than those of low energy level at the input or sendingend of the telephone circuit, whereby the differences of leveltransmitted over the circuit are made as small as is practicable. Thiscompression demands, of course, a corresponding expansion at the outputor receiving end of the telephone circuit in order that the normal rangeof energy level may be restored and transmitted.

Inthe drawing there is shown a transmission line, which it is understoodmay be a telephone line over which it is desired to transmit broadcastprograms. At the left or input end of this line, an arrangement of resistances FR and VR is connected through a transformer T, with thegrid-filament cir- 1929. Serial No. 374,920.

cuit of a vacuum tube amplifier A and the mission line, the resistancesFR and VH are .5

associated through the transformer T with the grid-filament circuit ofthe amplifier A and the plate-filament circuit of this amplifier isconnected through a transformer T to the output of the circuit.

The resistance FR- is a resistance of low temperature coefiicient. Theresistance VR is a resistance of high positive temperature coefficientand may take the form, as is indicated'on the'drawing, of a short, fine,tungsten filament in an evacuated bulb. Another form of resistance whichmay be employed is a hydrogen-iron ballast lamp designed to consume lessenergy than the ordinary commercial lamp of the type. VR presents arelatively low impedance to currents of low energy level and arelatively high impedance to currentsof'high energy level. The tworesistances FR and VB are connected in series as shown. It will be notedthat the voltage drop across the resistance FR is the one impressedthrough transformer T upon the input of amplifier A It will beunderstood by those skilled in the art that,

given this circuit arrangement, the apparatus at the input end of theline will function as a compressor of the range of energy level, givingless gain for the currents of high energy level than for the currents oflow energy level. In other words, the difference of energy level isconsiderably diminished.

At the receiving end of the line, the resistances FR and VH areconnected in series, as is the case at the sending end, but here thevoltage drop across the high temperature coefficient resistance VR isimpressed upon the input of amplifier A Since the voltage drop across VHis relatively greater for currents of high energy level than forcurrents of low energy level, the apparatus at this end of the circuitintroduces more gain for the high energy currents than for the lowenergy currents and accordingly acts as an expander of the range ofenergy level.

Given a line substantially free fromdistortion it Will be obvious tothose skilled in V s the art that the resistances FR ,VR ,FR and VR maybe so chosen that the combination of the compressor, the line and theexpander 5 Will introduce substantially no distortion.

'The ltrue'scope of the inventionis determined by the appended claims. Y

What is claimed is: in I s i 1; a transmission circuit, av fixed resist-I 'anceand a resistance varying with the energy level of currentsflowing through it connected in series atthe sending-end of. thecircuit, a

translating device at the sending end of the 'circuit, means forimpressing ontheinput of said translating device-the voltage drop"across sa'id fixed resistance,a'secondfixed resistanceanda secondresistance varying with the energy level of currents flowing through 1it connected in series at the receiving end of i thecircuit', a secondtranslating device at' the 1 receiving; end" o-fthe circuit, and meansfor impressing on the input of said second translatingdeViCe the voltagedrop across saidsec- V "ond vvaryi ng resistance." s c i D 2. In atransmission circuitytwo resist- "ances of substantially difli'erenttemperature coefilcientsi connected in series at the sending end of'thecircuit, a translating deviceat the sending end of the circuit, meansfor'impressingion the input of saidtranslatingdevice the voltage dropacross oneof said 'resistamies *two resistances ofsubstantiallydifierent tern- 5 perature c'oefiicients connected inseries at the ,7

receiving end of the circuit, a second translating device at thereceiving end ofthe circuit, U i

and means for impressing on the input of said i secondf translatingdevice thevoltage' drop across one of said: resistances'at the receivingend of the circuit, the resistance of which the f 4.0 voltage drop isutilized at the receiving end 1 being of relativelydifferenttemperatureco efiicient fro'm 'the: resistance of Whichvthe voltage drop isutilizedvat the sending end. a a In testimony whereof, I have signed myname to this specification this 28thday of June51929. "1 LELANDGLRQBERTS.

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